Fluidized bed method and reactor for the treatment of catalysts and catalyst carriers

ABSTRACT

A method for the treatment of catalysts or catalyst carriers by: a) introducing and distributing a gas in the lower section of a reactor containing a catalyst or catalyst carrier bulk material; b) forming a fluidized bed in the reactor; c) treating the particles in the fluidized bed while removing the fine particles an/or retaining the course particles by means of a separating organ and d) discharging the reactor. To this end, a reactor bottom which tapers downwards is used.

The present invention relates to a fluidized bed method, to a reactorfor the treatment of catalysts and catalyst carriers, and to the use ofthe method's products in the manufacture of polyolefins.

When gases flow from below through a bed of finely particulate materialsupported on perforated plates, a state similar to that of a boilingliquid becomes established under certain flow conditions—the bed throwsup bubbles, and the particles of the bed material are in constantswirling up and down motion within the bed and thus remain suspended toa certain extent. In this connection, the term fluidized bed is used.Such a state arises when a certain limiting value for the velocity ofthe gas flowing through the bed from below against the gravity of thesolid particles is reached. This point, at which the resting bed becomesa swirling bed, the fixed bed becomes a fluidized bed, is referred to asthe whirl or fluidizing point. The reaching of this point depends on anumber of physical factors; these are, for example, the density, size,distribution and shape of the particles and the properties of thefluidizing liquid.

Like a liquid, the fluidized bed can flow out through apertures, beconveyed through pipes or run off on inclined surfaces, for example aconveying channel. If the velocity of the fluidizing liquid is increasedfurther, the bed expands to an ever greater extent, and bubbles form.Above a limiting velocity, the particles are discharged from thecontainer as fluidized dust, but can be separated from the gas streamagain in a downstream separator and fed to the reactor.

An advantageous separator is a so-called cyclone. In such a cyclone, theseparation of the particles takes place with the aid of centrifugalforce. In principle, cyclones consist of a cylindrical vessel with aconically narrowing base into which an inlet pipe for dust-containingair projects tangentially and an outlet pipe for clean air projectsvertically. The gas/dust stream entering tangentially induces a whirlflow, with the relatively large dust particles being flung by thecentrifugal force against the wall of the cylinder and from there sinkto the base through the action of gravity, from where they can bedischarged. The circulating, dust-freed gas whirl reverses its directionat the base of the cyclone and leaves the cyclone in an upward directionthrough the outlet pipe, possibly together with finer particles, sincethe separation principle is not sufficient to remove fine-dustcontaminants completely. In industry, cyclones are predominantlyemployed for de-dusting.

Cyclones are an important component in many fluidized-bed processes.Fluidized-bed processes are used for a large number of industrialprocesses. The solid in the fluidized bed can act, for example, eitheras catalyst (fluidized-bed catalyst) or as heat-transfer agent, or canitself participate in the reaction. Important methods which can bedesigned as fluidized-bed methods are the following:

Gas-phase polymerization, coal combustion, coal liquefaction andFischer-Tropsch synthesis, catalytic cracking of hydrocarbons, roastingof sulfidic ores, calcination of hydrated aluminia, calcination oflimestone, desulfurization of gases, catalytic dehydrogenation ofbenzine-rich naphthene fractions, distillation of oil from bituminoussand, oxidation of naphthalene to phthalic anhydride on vanadium oxide,removal of fluorine in the recovery of phosphate, preparation ofacrylonitrile, dichloroethane, CCl₄, TiCl₄, drying of brown coal andgranular materials (for example comprising PVC, saltpeter, potassiumsalts, sawdust, sodium chloride, pigments, pharmaceutical preparations,insecticides, even microorganisms), aroma-retaining drying or roastingof foods (coffee beans, cocoa, groundnuts, cereal products, corn starch,rice, tea and many others), incineration of waste, garbage, specialwaste and sewage sludge, or physical processes, such as separation ofsmall particles or mixing.

Fluidized-bed methods for the treatment of catalysts, initiators,catalyst carriers, initiator carriers and of carrier materials treatedwith initiators or catalysts will be considered below. Although strictlyspeaking a differentiation must be made in definition between initiatorsand catalysts, the term “catalysts” below is also taken to meaninitiators (it is frequently only possible to tell with difficultywhether a reaction is initiated or catalyzed). Correspondingly, carriermaterials treated with active components—for example catalysts—are alsoreferred to as catalysts below.

For treatment of catalyst carriers or catalysts (for example for usethereof in the polymerization of olefins), use is made of fluidized-bedmethods in which the particles are moved by an upwardly directed gasstream with which they are in intense material and heat exchange. Astime passes during the process, the particles are heated and undergo aphysical/chemical change. When the conversion is complete, the particlesare cooled and discharged from the reactor. In designing the reactors,particular attention is paid to the following:

-   A) gas distribution at the inlet-   B) removal of fine particles from the gas stream leaving the reactor-   C) discharge device    Regarding A) (Gas Distribution)

The gas is distributed using flat, curved or inclined plates in thelower region of the reactor, the plates being provided with varioustypes of passages for the gas. In the simplest case, these passages areholes, but can also be suitable inserts, for example bells or screws.For reliably uniform distribution of the gas, a gas distribution plateof this type requires a pressure loss of between at least 10 and 20mbar. Advantageous distribution of the gas may be prevented by thepassages for the gas being blocked by the particles.

In some beds, consisting of particles of certain materials and size, thephenomenon of channel formation is observed during introduction of gasinto one of the reactors described. In this case, a fluidized bed is notformed, but instead the gas flows through the plate passages verticallyupward through the bed. Even if the gas flow rate is increased, theparticles then remain in the bed.

Distribution of the gas also plays a role in the treatment of particleswith liquids sprayed into the fluidized bed. These liquids can function,for example, as binders for the particles, which thus aggregate and formrelatively large agglomerates during drying.

Optimum distribution of the sprayed-in liquid by the fluidizing gas iscrucial in avoiding firstly agglomeration of solid particles andsecondly deposits forming on the reactor walls due to coating by solid[Daizo Kunii, Octave Levenspiel, “Fluidization Engineering”,Butterworth-Heinemann (Stoneham), second edition (1991), p. 24].

Regarding B) (Separation)

At the reactor outlet, the gas is passed through a suitable separator,by means of which entrained particles are removed in order to keep themin the reactor. Such separators can be filter elements suspendeddirectly in the reaction space. The disadvantage of these filterelements is that they become blocked and therefore must be cleaned orreplaced regularly. It is advantageous to use a cyclone separator, whichis essentially maintenance-free and, in contrast to a filter, has theability to allow very fine particles to leave the reactor and reliablyto retain relatively large particles. This property can have a positiveeffect on quality of the fluidized bed produced, since very fineparticles are often undesired in later use. Microfine catalyst particlescan, for example, cause so-called hot spots, which are undesired inlater polymerizations.

Regarding C) (Discharge)

When the treatment is complete, the catalyst or catalyst carrier isdischarged from the reactor via valves to be closed suitably. Theopenings are formed in the plate in order to minimize the amount ofparticles remaining in the reactor. The catalyst or catalyst carriermust necessarily pass through the passages of the gas distribution plateduring the discharge process. The gas should be able to continue to flowthrough the plate in order to ensure the mobility of the particles (thelatter do not automatically “slide” to the outlet, so it is necessary touse the fluidizing gas during discharging). However, the use offluidizing gas during discharging hinders the advantageous use of acyclone separator:

During discharging, the level of the fluidized bed drops to below theend of the outlet pipe of the cyclone, and, owing to the short-circuitgases consequently formed, the separation efficiency of the cyclone isgreatly reduced, meaning that even relatively large particles aredischarged from the reactor. This inevitably results in loss ofmaterial.

A further disadvantage is that the reactors described above cannot beemptied completely, since material always remains on the plates. Theresidues are passed through again together with fresh particles, givingmaterial with a variety of residence times. This is generally of uneven,usually worse quality than material having a uniform residence time.

It is an object of the present invention to improve the above-describedfluidized-bed method for the treatment of catalysts or catalyst carriersin such a way that channel formation does not occur, advantageous use ofa cyclone is possible, and rapid and at least virtually complete, i.e.residue-free, discharging of the reactor takes place.

We have found that this object is achieved by a method for the treatmentof catalysts or catalyst carriers by

-   a) introducing and distributing gas in the lower section of a    reactor containing a catalyst or catalyst carrier bulk material,-   b) forming a fluidized bed in the reactor,-   c) treating the catalyst or catalyst carrier particles in the    fluidized bed, and-   d) discharging the reactor,    using a reactor bottom tapering downwards.

In a preferred embodiment, relatively fine particles are removed and/orrelatively large particles are retained by means of a separator.

In accordance with the invention, provision is also made for anapparatus for carrying out this latter process which comprises thefollowing devices:

-   i) a reactor jacket having a reactor bottom tapering downwards,    preferably conical,-   ii) a pipe for introducing gas into the reactor located beneath the    reactor bottom and connected to a gas inlet pipe for gas    introduction,-   iii) a device for discharging the reactor located beneath the    reactor bottom, and-   iv) one or more separators.

The inventive solution, the provision of a reactor bottom taperingdownwards for gas distribution in the basic fluidized-bed method, ispresumably based on the fact that, surprisingly, the particles to betreated undergo virtually no damage or deactivation in the process.Downwardly tapering reactor bottoms are taken to mean those whosecross-sectional area reduces in a downward direction. In principle,symmetrical and asymmetrical shapes are possible. For example, atruncated pyramid, but in particular a truncated cone—i.e. a conicalreactor bottom—is suitable. If these reactor bottoms tapering downwardsare used, a layer located in the lower region of the bottom andsurrounding the inside of the jacket is always present in addition tothe fluidized bed. Exchange of material takes place in the layer, withparticles of the fluidized bed entering the layer and on the other handmaterial leaving the layer due to “sliding off” of particles into thegas-introduction region and being fed back to the fluidized bed. Due toheat transfer from the reactor wall to the layer, undesirably hightemperatures can occur in the latter. Possible consequences would be,for example, deactivation of the catalyst or sintering processes causingthe formation of agglomerates and/or the blocking of the pores of thecatalyst or catalyst carrier.

These disadvantages virtually do not occur, or do not occur at all, inthe method according to the invention—possibly because the catalystparticles remain in the layer for only a very short time.

The layer acts advantageously in that it prevents channel formation (theconstant sliding of particles off the jacket wall would immediatelyclose each “channel”). In addition, the layer, owing to its conicalstructure, favors uniform distribution of the fluidizing gas.

The attached drawing shows in FIG. 1 a gas distribution plate 1, areactor discharging device 3, filter elements 5, a gas outlet 6, a gasinlet 7 and in FIG. 2 a reactor bottom 2, a reactor discharging device3, a cyclone 4, a gas outlet 6, a gas inlet 7, a truncated cone 8, apipe 9 for gas introduction into the reactor, a cone angle α and anangle β.

Since the reactor according to the invention (FIG. 2), in contrast tothe reactor type usually used hitherto (FIG. 1), does not have a gasdistribution plate 1, firstly the pressure loss which would beassociated therewith does not occur, and secondly discharging of thereactor is simplified and is carried out without 2, the use offluidizing gas. The conical reactor bottom 2 enables catalyst orcatalyst carrier to be removed from the reactor with greater efficiency,since the catalyst or catalyst carrier slides off the wall and all orvirtually all reaches the discharge device 3 without leaving significantresidues in the reactor. The reactor can thus be discharged residue-freeor virtually residue-free (i.e. to the extent of at least 99%,preferably to the extent of at least 99.5%). It is advantageous for theconical reactor bottom 2 of the reactor to have a cone angle α, measuredbetween the two internal jacket surfaces, of from 10° to 120°,preferably from 30° to 80°. The discharge device 3 (for example a pipe)is generally located at the lower end of the pipe 9 serving for gasintroduction into the reactor. The pipe 9 thus partly also fulfills afunction for discharging of the reactor. The discharging generally takesplace significantly more quickly than in corresponding reactors having agas distribution plate 1.

In order to remove entrained particles, the head of the reactor can havea cross-sectional widening. Additional separators can be installed inparticular in the region of this widening.

A further essential advantage of the reactor according to the inventionis that the separator used can advantageously be a cyclone 4, i.e.effective and reliable discharge of fine material is facilitated withouthaving to accept material losses during discharging of the reactor. Thedisadvantages of the filter elements 5 employed in the processes usuallyused hitherto, which are located beneath the gas outlet 6, have beendescribed in the introduction. The separator used in all cases serves toremove relatively fine particles and/or to retain relatively largeparticles.

Also of importance is the introduction of the carrier gas at the gasinlet 7. Since, as far as possible, no particles should enter the gasinlet 7 during charging and discharging, the corresponding inlet pipeshould be inclined upward. The angle β measured between the gas inletpipe of the gas inlet 7 and the upward verticals is, in particular, from20° to, 70°, preferably from 30° to 60°.

The catalysts or catalyst carrier treated in the process according tothe invention are employed, in particular, in the polymerization ofolefins, in which case the particles to be treated are generally fed tothe reactor in the form of solid particles. Such polyolefin catalystsfrequently contain doped carrier materials (for example based on silicagel). The active components used are, for example, transition metals,such as chromium or titanium. Examples of carrier materials are oxidiccompounds, such as silica, alumina, silica-alumina, zirconia, thoria,fluorinated silica, fluorinated alumina, fluorinated silica-alumina,boron oxides or mixtures thereof. An additional surface modification ofthe carrier materials may be particularly advantageous. The treatment ofthe catalysts or catalyst carriers is generally a calcination and/oractivation.

During the treatment, in addition to the carrier gas (fluidizing gas)introduced through the gas inlet 7, additional gases and, in addition tothe originally introduced particles, additional solid can also beintroduced into the fluidized bed. This introduction can take place atany time during the process and through feed points installed at anydesired locations. Examples of suitable additional gases are oxygen,carbon dioxide or steam, while examples of additional solids which canbe employed are ammonium hexafluorosilicate, untreated catalyst carriersor catalysts having a different physical/chemical structure. Inaddition, liquids, for example water, can be sprayed into the fluidizedbed. Thus, liquids, additional solids and/or additional gases can alsobe introduced into the reactor.

The treatment by the method according to the invention is described ingreater detail below with reference to working examples.

EXAMPLE 1 (CALCINATION)

25 kg of catalyst carrier having a bulk density of 450 kg/m³ and aparticle size distribution as shown in Table 1 were treated in a steelreactor having an overall height of 4 m, a diameter (cylindrical) of 0.3m, a cone angle of 45° and an internal diameter of the pipe 9 installedon the truncated cone 8 of 25 mm. The reactor was heated from ambienttemperature to 600° C. over the course of 6 hours, with N₂ being used asfluidizing gas. The reactor was subsequently held at this temperaturefor 10 hours and then cooled. The velocity, based on the empty pipe, inthe cylindrical reactor part was between 4 cm/s and 8 cm/s. After theend of the process, the fluidizing gas was turned off and the catalystsupport discharged. After the emptying process, about 0.05 kg ofcatalyst carrier (i.e. about 0.2%) remained in the reactor adhering tothe wall as a dust coating. TABLE 1 Material properties of the silicagel ES70X ® Test Pore volume 1.69 cc/g Surface area 320 m²/g Volatilecontent 7.0% Soda (as Na₂O) 500 ppm Bulk density 300 g/l

Particle size distribution of the silica gel ES70X® (manufacturerCrosfield Catalysts) before and after calcination Median MaterialTreatment μm <20.2 μm <32 μm >80.7 μm ES70X ® untreated 40.0 1.0 18.50.2 (silicate Heating for 40.5 1.1 19.1 0.1 with about 10 h at 99.3%SiO₂ 600° C. under N₂(Measurement method: Coulter counter, pre-treatment: 30 sec ultrasound,electrolyte: 49.5% water, 49.5% glycerol, 1% NaCl, capillary: 560 μm,operating mode: manual)

EXAMPLE 2 (ACTIVATION)

200 kg of catalyst having a bulk density of 420 kg/m³ and a particlesize distribution as shown in Table 2 were activated in a steel reactorhaving an overall height of 5 m, a diameter (cylindrical) of 0.6 m, acone angle of 45° and an internal diameter of the pipe 9 installed onthe truncated cone 8 of 51 mm. The apparatus was heated from ambienttemperature to 705° C. over the course of 10 hours, with air being usedas fluidizing gas. The apparatus was subsequently held at thistemperature for 10 hours and then cooled. During the cooling phase, thefluidizing gas was switched to nitrogen. The velocity, based on theempty pipe, in the cylindrical reactor part was 5 cm/s-10 cm/s. Afterthe end of the process, the fluidizing gas was turned off and thecatalyst discharged. After the emptying process, about 0.1 kg ofcatalyst (i.e. about 0.05%) remained in the reactor.

COMPARATIVE EXAMPLE C2 (ACTIVATION)

125 kg of catalyst of the type from Example 2 were activated in areactor having an overall height of 5.5 m, a diameter of 0.6 m and ahorizontal gas distribution plate with cylindrical holes (perforatedplate). The apparatus was heated from ambient temperature to 705° C.over the course of 10 hours, with air being used as fluidizing gas. Theapparatus was subsequently held at this temperature for 10 hours andthen cooled. During the cooling phase, the fluidizing gas was switchedto nitrogen. The velocity, based on the empty pipe, in the cylindricalreactor part was 5 cm/s-10 cm/s. After the end of the process, thecatalyst was discharged via a centrally installed outlet pipe. After theemptying process, 5.2 kg (i.e. about 4%) remained on the distributionplate. TABLE 2 Material properties of the catalyst Sylopol 969 IDW ®Commercial Test product After activation Pore volume, cc/g 1.24 1.24Surface area, m²/g 316 not determined Volatile content, % 6.1 notdetermined Na₂O, % 0.08 not determined Bulk density, g/l not determined329

Particle size distribution of the catalyst Sylopol 969 IDW®(manufacturer Grace GmbH) before and after activation Median MaterialTreatment μm <20.2 μm <32 μm >80.7 μm Sylopol untreated 56.2 0.8 8.314.1 969 IDW ® Heating for 51.5 0.8 9.0 5.8 (silicate 10 h at with about705° C. 98% SiO₂ and about 1% Cr

(Measurement method: Coulter counter, pretreatment: 30 sec ultrasound,electrolyte: 49.5% water, 49.5% glycerol, 1% NaCl, capillary: 560 μm,operating mode: manual).

1-5. (canceled)
 6. A reactor comprising the following devices: i) areactor jacket comprising a reactor bottom which tapers downwards, ii) apipe for introducing gas into the reactor located beneath the reactorbottom and connected to a gas inlet pipe for gas introduction, iii) adevice for discharging the reactor located beneath the reactor bottom,and iv) a separator.
 7. A reactor as claimed in claim 6, wherein thereactor bottom is conical.
 8. A reactor as claimed in claim 7, whereinthe conical reactor bottom has a cone angle α, measured between the twointernal jacket surfaces, of 10° to 120°.
 9. A reactor as claimed inclaim 6, wherein an angle β measured between a gas inlet pipe of the gasinlet and the upward verticals is 20° to 70°.
 10. A reactor as claimedin claim 6, wherein the separator is a cyclone.
 11. (canceled)
 12. Areactor as claimed in claim 8, wherein said cone angle α is 30° to 80°.13. A reactor as claimed in claim 9, wherein said angle β is 30° to 60°.